The present invention disclosed herein relates to electrodes including a nanostructure, and more particularly, to electrodes using catalytic metal having a nanocup or nanoring structure in which a structure of catalytic metal usable in a fuel cell or a dye-sensitized solar cell is improved.
A nanostructure denotes a structure having a nanoscale dimension, i.e., a size of approximately 10−9 m, a nanometer unit. The nanostructure exhibits various physical and chemical characteristics, which could not be discovered in a conventional material, such as nanoscale dimension, quantum confinement effect, superior crystallinity, and high surface area to volume. Many attempts have been made to apply nanostructures exhibiting various electrical or optical characteristics according to their sizes or shapes to typical electrochemical devices or optical devices, and as a result of such research and development, devices may be miniaturized as well as being advanced. Despite the wealth of technological advancements to date, research and development for improving performances of various metal nanostructures and various devices using the metal nanostructures have currently been undertaken.
Meanwhile, a dye-sensitized solar cell (DSSC) is an energy conversion device that converts light energy into electrical energy by the transfer of electrons and holes generated in a dye having light energy absorbed therein through the application of the photosynthesis process of plants. When referred to FIG. 1A schematically showing structure and power generation principle of a typical dye-sensitized solar cell, the dye-sensitized solar cell has a sandwich structure of two transparent substrates. The cell is composed of a transparent electrode coated on a transparent substrate, semiconducting oxide composed of nanoparticles adhered to the transparent electrode, a dye polymer coated in a monomolecular layer on surfaces of the semiconducting oxide particles, an electrolyte solution filling a space having a thickness range of 30 μm to 100 μm between two electrodes, and a counter electrode. When sunlight is absorbed in the semiconducting oxide electrode having dye molecules chemically adsorbed to the surface thereof, dye molecules generate electron-hole pairs, and electrons are injected into a conduction band of the semiconducting oxide and then move to the transparent conductive layer through interfaces between nanoparticles and the counter electrode through an external conducting wire. In the dye-sensitized solar cell, one surface of the counter electrode in contact with the electrolyte is coated with catalytic metal as a way of promoting a reduction reaction of the electrolyte. With respect to the dye-sensitized solar cell, the reduction reaction of the electrolyte is performed at the counter electrode and hereinafter, in the case that the electrode of the present invention is referred to as “reduction electrode” in relation to the dye-sensitized solar cell, the electrode is defined to refer to “counter electrode” of the dye-sensitized solar cell.
Different from the dye-sensitized solar cell, a fuel cell is an energy conversion device that directly converts chemical energy from a fuel into electrical energy through a chemical reaction. When referred to FIG. 1B schematically showing structure and power generation principle of a typical fuel cell, the fuel cell is composed of an electrolyte inserted between an oxidation electrode and a reduction electrode, and an oxidation reaction of hydrogen occurs at the oxidation electrode and a reduction reaction of oxygen occurs at the reduction electrode. When the power generation principle of the fuel cell is described in more detail, electrons and hydrogen ions are generated as hydrogen is oxidized at the oxidation electrode. The generated electrons and hydrogen ions are transferred to the reduction electrode respectively through an external circuit and the electrolyte, and then an entire circuit is completed as a reduction reaction of the transferred hydrogen ions and electrons with oxygen supplied from the reduction electrode occurs to generate water. Therefore, an electric potential of the cell obtained through the fuel cell is defined as a difference between an electric potential generated when hydrogen is oxidized at the oxidation electrode and an electric potential generated when oxygen is reduced at the reduction electrode. In the fuel cell, one surface of the oxidation electrode or the reduction electrode in contact with the electrolyte is coated with catalytic metal as a way of promoting an oxidation/reduction reaction of the oxidation electrode or the reduction electrode.
In various cells generating electrical energy by using an oxidation/reduction reaction including the reduction electrode of the dye-sensitized solar cell or the oxidation or reduction electrode of the fuel cell, catalytic metal coated on one side of the electrode to promote the oxidation or reduction reaction is generally coated by using a physical or electrochemical deposition method. In the case that catalytic metal is coated by using the physical or electrochemical deposition method, the catalytic metal is coated in the shape of a flat thin film on the electrode. In view of surface area, with respect to the catalytic metal having the shape of the flat thin film, an area having a catalytic reaction generated therein may be limited. That is, with respect to the catalytic metal having a typical flat thin film structure, an area, in which electrons can move during an oxidation or reduction reaction, is limited to an area of the flat metal thin film. Such structural limitation of the catalytic metal must be addressed to improve a power generation efficiency of the cell. However, a technique for addressing the limitation has not been suggested to date.